git-merge documentation: conflicting merge leaves higher stages in index

git-merge documentation: conflicting merge leaves higher stages in index

This hopefully concludes the latest updates that changes the
behaviour of the merge on an unsuccessful automerge.  Instead of
collapsing the conflicted path in the index to show HEAD, we
leave it unmerged, now that diff-files can compare working tree
files with higher stages.

Signed-off-by: Junio C Hamano <junkio@cox.net>